Interpret subprocess output as strings

Previously the subprocess output would be reported as byte strings,
which in Python 3.10 would add a b'' prefix, interfering with the
output. By interpreting them as proper UTF-8 strings, we ensure
that the original functionality is restored.

deployment/packer/driver.py

@@ -59,7 +59,9 @@ def get_git_sha():
                    'rev-parse',
                    'HEAD']
 
-    return subprocess.check_output(git_command).rstrip()
+    return subprocess.check_output(
+        git_command,
+        universal_newlines=True).rstrip()
 
 
 def get_git_branch():
@@ -69,7 +71,9 @@ def get_git_branch():
                    '--abbrev-ref',
                    'HEAD']
 
-    return subprocess.check_output(git_command).rstrip()
+    return subprocess.check_output(
+        git_command,
+        universal_newlines=True).rstrip()
 
 
 def get_git_desc():
@@ -81,7 +85,9 @@ def get_git_desc():
                    '--dirty',
                    '--abbrev=40']
 
-    return subprocess.check_output(git_command).rstrip()
+    return subprocess.check_output(
+        git_command,
+        universal_newlines=True).rstrip()
 
 
 def run_packer(mmw_config, machine_types, aws_profile):